1. Field of the Invention
The present invention relates to a thin film magnetic memory device and, more specifically, to a randomly accessible thin film magnetic memory device including a memory cell having a magnetic tunneling junction (MTJ).
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device is recognized as a memory device capable of non-volatile data storing with low power consumption. The MRAM device performs non-volatile data storing using a plurality of thin film magnetic elements formed on a semiconductor integrated circuit, and is randomly accessible for each thin film magnetic element.
Recently, it is reported that the performance of the MRAM device is dramatically improved when a tunneling magneto-resistance element utilizing a magnetic tunnel junction (MTJ) is used as a memory cell. The MRAM device including a memory cell having a magnetic tunnel junction is disclosed in references such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7.2, February 2000, “Nonvolatile RAM Based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000, and “A 256kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 25 schematically shows a structure of a memory cell (MC) having a magnetic tunnel junction (also referred to as an MTJ memory cell hereinafter).
Referring to FIG. 25, the MTJ memory cell includes a tunneling magneto-resistance element TMR which changes in an electric resistance value corresponding to a data level of stored data, and an access transistor ATR. Access transistor ATR is formed with a field-effect transistor, and is coupled between tunneling magneto-resistance element TMR and a ground voltage VSS.
A write word line WWL for directing data writing, a read word line RWL for directing data reading, and a bit line BL which is a data line for transferring an electric signal corresponding to a level of stored data at data reading and data writing are arranged in the MTJ memory cell.
FIG. 26 is a schematic diagram showing a data reading operation from the MTJ memory cell. First, a structure of tunneling magneto-resistance element TMR is described.
Referring to FIG. 26, the tunneling magneto-resistance element has an MR (Magneto-Resistive) effect, with which an electric resistance of a substance is changed corresponding to a direction of magnetization of a magnetic element. Tunneling magneto-resistance element TMR is characterized in that, it has a significant MR effect even at ordinary temperatures, and has a high MR ratio (an electric resistance ratio corresponding to the direction of magnetization).
Tunneling magneto-resistance element TMR includes ferromagnetic films FL and VL and an insulator film (tunneling film) TB. In tunneling magneto-resistance element TMR, an amount of tunneling current flowing through insulator film TB which is sandwiched between ferromagnetic films FL and VL changes with a direction of electron spin, which direction is determined by the directions of magnetization of ferromagnetic films FL and VL. Because the number of states possible for the spinning electrons within ferromagnetic films FL and VL differs depending on the directions of magnetization, the tunneling current increases when ferromagnetic films FL and VL have the same direction of magnetization, while the tunneling current decreases when these two films have opposite directions of magnetization.
Utilizing this phenomenon, the tunneling magneto-resistance element TMR can be used as a memory cell storing 1 bit data, when the direction of magnetization of ferromagnetic film FL is fixed while the direction of magnetization of ferromagnetic film VL is changed corresponding to the stored data, and the amount of the tunneling current flowing through tunneling film TB, i. e., the electric resistance of tunneling magneto-resistance element TMR is detected. The direction of magnetization of ferromagnetic film FL, which is fixed by an antiferromagnetic material or the like, is commonly referred to as “a spin valve”.
Hereinafter, ferromagnetic film FL having a fixed direction of magnetization is also referred to as a fixed magnetic film FL, and ferromagnetic film VL having a direction of magnetization corresponding to stored data is also referred to as a free magnetic film VL. In tunneling magneto-resistance element TMR, the side of free magnetic film VL which is electrically coupled to bit line BL is also referred to as a positive electrode (+), and the side of fixed magnetic film FL which is electrically coupled to access transistor ATR is also referred to as a negative electrode (−).
On data reading, access transistor ATR is turned on in response to activation of read word line RWL. With this, a sense current Is, which is fed as a constant current from a control circuit, not shown, flows through a current path from bit line BL to tunneling magneto-resistance element TMR, access transistor ATR, and ground voltage VSS.
As described above, the electric resistance value of tunneling magneto-resistance element TMR changes corresponding to a relation between the directions of magnetization of fixed magnetic film FL and free magnetic film VL. More specifically, when the direction of magnetization of fixed magnetic film FL is the same as the direction of magnetization written to free magnetic film VL, the electric resistance value of tunneling magneto-resistance element TMR will become smaller than the situation when they have different directions of magnetization. In this specification, the electric resistance values of the tunneling magneto-resistance element corresponding to “1” and “0” of stored data will be indicated by Rmax and Rmin, respectively. Herein, it is assumed that Rmax>Rmin, and that Rmax=Rmin+ΔR.
Thus, the electric resistance value of tunneling magneto-resistance element TMR changes corresponding to the magnetic field applied from the outside. Therefore, data storing can be performed based on the change characteristic of the electric resistance value of tunneling magneto-resistance element TMR. Generally, the electric resistance value of tunneling magneto-resistance element TMR applied to the MRAM device is about several tens Ω.
The change in voltage in tunneling magneto-resistance element TMR, which change is generated by sense current Is, differs corresponding to the direction of magnetization stored in free magnetic film VL. Therefore, the level of stored data of the MTJ memory cell can be read by monitoring the change in voltage level of bit line BL, if the feeding of sense current Is is started after bit line BL is precharged to a high voltage.
FIG. 27 is a schematic diagram showing a data writing operation to the MTJ memory cell.
Referring to FIG. 27, when data is written, read word line RWL is deactivated and access transistor ATR is turned off. In this state, a data write current for writing the magnetic field to free magnetic film VL flows through write word line WWL and bit line BL, respectively. The direction of magnetization of free magnetic film VL is determined by the combination of the directions of the data write current (+Iw or −Iw) respectively flowing through write word line WWL and bit line BL.
FIG. 28 is a schematic diagram showing a relation between the direction of data write current and the direction of magnetization during data writing.
Referring to FIG. 28, a magnetic field Hy shown by a vertical axis indicates a direction of a magnetic field H(BL) generated by the data write current flowing through bit line BL. On the other hand, a magnetic field Hx shown by a horizontal axis indicates a direction of a magnetic field H(WWL) generated by the data write current flowing through write word line WWL.
The direction of magnetization stored in free magnetic film VL is newly written only when the sum of magnetic fields H(BL) and H(WWL) reaches the outer region of the asteroid characteristic line shown in the drawing. That is, the direction of magnetization stored in free magnetic film VL is not updated when the magnetic field is applied which corresponds to the inner region of the asteroid characteristic line.
Therefore, the current must flow through both write word line WWL and bit line BL to update the stored data of tunneling magneto-resistance element TMR by the writing operation. The direction of magnetization once stored in tunneling magneto-resistance element TMR, i. e., the stored data is held in non-volatile manner until new data writing is performed.
Sense current Is also flows through bit line BL during the data reading operation. There is only a slim possibility, however, to wrongly rewrite the stored data of the MTJ memory cell on data reading by the effect of sense current Is, because sense current Is is generally set to be smaller than the data write current described above by about one or two orders of magnitude.
In the above-described references, a technique is disclosed to form an MRAM device, which is a random access memory, by integrating such MTJ memory cells on a semiconductor substrate.
FIG. 29 is a schematic diagram showing MTJ memory cells integrated and arranged in rows and columns.
Referring to FIG. 29, a highly-integrated MRAM device can be implemented by arranging the MTJ memory cells in rows and columns on a semiconductor substrate. In FIG. 29, the MTJ memory cells are arranged in n rows×m columns (n, m: natural numbers). For the n ×m MTJ memory cells arranged in rows and columns, n write word lines WWL1-WWLn and read word lines RWL1-RWLn as well as m bit lines BL1-BLm are arranged.
On data reading, one of read word lines RWL1-RWLn is selectively activated, and the memory cells belonging to the selected memory cell row (also referred to as “selected row” hereinafter) are electrically coupled between respective bit lines BL1-BLm and ground voltage VSS. As a result, the amount of current passing through each of bit lines BL1-BLm changes according to a stored data level of the corresponding memory cell.
Therefore, the stored data level of the selected memory cell can be read by comparing the passing current of the bit line corresponding to the selected memory cell column with a prescribed reference passing current using a sense amplifier or the like.
A dummy resistance is generally used to generate the reference passing current.
It is desirable to set the dummy resistance to the electric resistance value corresponding to the intermediate value between Rmax and Rmin, which are electric resistance values corresponding to the stored data of the memory cell selected to generate a desired passing current as a reference passing current.
To set the dummy resistance to the intermediate electric resistance value, however, a certain manufacturing process is needed, so that the process will be complicated. It is also needed to consider the variation due to the manufacturing process. Therefore, it is difficult to manufacture the dummy resistance in a simple manner.
Though there is a method of designing an ideal intermediate electric resistance value using a dummy cell as the dummy resistance, the electric resistance value of the dummy cell varies due to the voltage applied to opposite ends of the dummy cell, that is, a bias voltage. Thus, because a tunneling magneto-resistance element forming the dummy cell has a voltage dependency, there has been a situation wherein an actual electric resistance value of the dummy cell differs from the ideal intermediate electric resistance value, which made it difficult to generate the reference passing current with high precision. As a result, it has been difficult to perform high-speed and stable data reading.